In the past various circuits have been employed in association with frequency generation in an ultrasonic cleaning device. Some of these prior art devices generate ultrasonic vibrations using a half bridge square wave amplifier to drive a radio frequency transformer, which in turn provides a feedback to control the half bridge squarewave amplifier. Most cleaning devices include a plurality of piezoelectric ultrasonic transducer physically connected to the underside of a thin metal bottom of a tank containing liquid for cleaning objects in the tank. If a plurality of such transducers are employed, they are connected in parallel across the output leads of the RF transformer. In addition, a feedback network is provided to derive signals from the RF transformer and operate the half bridge amplifier.
Certain design features of conventional frequency generators contribute to specific deficiencies in these devices, however. Specifically, the output frequency of the RF transformer is not uniform, but varies considerably. Since a piezoelectric ultrasonic cleaning tank is a difficult type of a electric load to power, the impedance of the piezoelectric transducers associated with the tank varies rapidly and in a complex way with frequency, with the level of fluid in the tank, and with size and shape of the work pieces in the fluid.
It is desirable to stablize the RF transformer output frequency yet minimize the power required to operate the system. Accordingly, some ultrasonic cleaners have been designed to be driven at minimum electrical impedance, which occurs at approximately the frequency of mechanical resonance between the mass of mechanical and structural parts external to the piezoelectric transducers and the stiffness of the intergal parts of the transducer. This is the resonant frequency of the system. Other systems are designed for operation at a slightly higher frequency where the force needed to move the mass of the external parts is greater than the force needed to overcome the stiffness of the crystal assembly. Electrically, this appears as though the transducer is inductive. At the same time, a gnerator must supply current to the capacitance of the piezoelectric crystals. Where the inductive and capacitive currents are equal, the transducer impedance is at a maximum. The frequency at which this occurs is called anti-resonance, as distinguished from mechanical resonance.
It is extremely difficult to maintain operation of a generator at either resonance or anti-resonance, however. Both the resonant and anti-resonant frequencies are quite unstable and vary significantly in the dynamic operation of an ultrasonic transducer system. Thus, the impedance at particular frequencies determined to be the average resonance and anti-resonance frequencies is likewise unstable and varies rapidly. It has therefore been the practice in some prior systems to operate the frequency generator above the anti-resonance frequency in a region some distance from the fast changing resonance and anti-resonance peaks, but not so high as to require excessive voltage.
To operate at frequencies above anti-resonance however, conventional systems have employed component inductors and capicators of large capacity and at considerable expense. These components are subjected to high voltage stresses and, accordingly, tend to overheat or arc over. In these prior systems, instability and noise are improved at the expense of requiring physically larger inductances and inordinantly high voltage capacitance.
It is an object of the present invention to provide an improved circuit design for an ultrasonic frequency generator operating at a frequency above anti-resonance. The improvement is achieved by providing a feedback network that is connected in series with the circuitry powering the piezoelectric crystals and by aleviating the requirement of conventional systems for the large capacitor and inductor that are associated with the crystals.
It is also an object of the invention to provide a feedback network in an oscillatory circuit for an ultrasonic generator employing a half bridge squarewave amplifier that compensates for fluctuations in frequency and tends to drive the transducer frequency toward a predetermined target frequency. In providing feedback signals to a half bridge squarewave amplifier, the phase of the feedback signals must lead the amplifier output slightly since there is a lag through the transistors of the amplifier. If the frequency output to the transucers begins to increase above the target frequency, however, retarding the advance of the phase of the feedback tends to bring the frequency back down to the target frequency. Conversely, if the frequency through the transducers begins to drop, a phase advance in feedback to the amplifier brings the frequency of the output thereof back up to the target frequency. The circuitry of the present invention, provides the feature of centering the frequency output of the amplifier about a predetermined target frequency in a self-correcting manner.